Over voltage relay



July 31, 1962 R. c. MAUER OVER VOLTAGE RELAY Filed March 15, 1957 Agent3,047,776 OVER VOLTAGE RELAY Robert C. Mauer, Sunland, Calif., assignorto Lockheed Aircraft Corporation, Burbank, Calif. Filed Mar. 15, 1957,Ser. No. 646,335 2 Claims. (Cl. 317-31) The present invention relates tovoltage responsive switches or relays and, more paiticularly, to avoltage responsive switch relay employing semi-conductors sultable foruse as an over voltage relay in electric current generating systems.

Over voltage relays are commonly used in alternating and direct currentgenerating systems to perform a switching function resulting in shuttingolf a generator causing the over voltage. A relay for use in this typeof system requires a time delay in its operation upon the currents of anover voltage so as not to be operated by unobjectionable transits in thesystem. The relay should also have an inverse time characteristic sothat the delay in the operation of the contact decreases as themagnitude of the over voltage increases.

One of the important objects of the present invention is to provide anew and improved over voltage relay in which the over voltage isdetermined by the attainment of a predetermined threshold voltage and inwhich there is a time delay in the operation of the relay upon theoccurrence of an over voltage, the duration of the time delay normallydecreasing as the magnitude of the over voltage increases.

Another object of the present invention is to provide an over voltagerelay or switch for aircraft electrical systems, for example, which issubstantially unaffected by the wide range of operating temperatures towhich aircraft components are sometimes subjected.

Another object of the present invention is to provide a new and improvedover voltage circuit which produces a signal that is substantiallynon-linear with respect to variations of input voltage. The device ofthe present invention provides a reference diode network in which theassociated nonlinear signal is used as the basic sensing element for theover voltage.

Still a further object of the present invention is to provide anon-linear diode circuit responsive to an over voltage for generating acontrolling signal which operates a relay. The relay is voltagesensitive rather than current sensitive.

With reference to the accompanying schematic drawing, there is shown adirect current over voltage operated relay in accordance with thepresent invention. Basically, the relay circuit of the present inventioncomprises a relay having a solenoid 11 which when energized closes aplurality of relay contacts such as the contacts 12 and 13, a transistor14 coupled to the solenoid for current amplification and supplying thesolenoid with operating current and a network of semiconductors such asdiodes D1, D2 and D3 arranged in a cascade relationship which suppliedthe transistor with operating current proportional to the value of overvoltage, the value of over voltage being the voltage above the normalvoltage. The diodes are characterized by their ability to pass currentin their reverse direction when a specied voltage is applied. Thiscurrent is a signal current which when applied to the transistor andamplified causes the solenoid to energize and the relay contacts to pullin. The characteristics of the diodes and the transistor amplicationcharacteristics provide a relay solenoid current varying from zero to apull in level of the relay for an input voltage variation of 1.4 volts.That is, if the relay is set to operate at a positive 32 volts, forexample, no voltage is applied to the solenoid until 30.60 volts havebeen reached. For purposes of clarification and 3,047,776 Patented July31, 1962 convenience of description, the present invention will bedescribed according to various functions of the circuit.

Diode Network Assuming that the normal voltage placed across a pair ofinput terminals 15 and 16 is 32 volts and that relay 1t) wil-l energizeat 33 volts. The path to a ground connection 17 from input terminal 15is via resistors R1, R2 diode D1 and D2, and resistors R6, R7 and R8. Aportion of this path is shunted by resistors R3 and R5 which divide the32 volt input. Selection of proper values for the resistors and thediodes should be made so that approximately 12 volts appear at the topof diode D1. For clarification, the top of a diode or the top of aresistor is that point of highest voltage for that particular diode orresistor. When there are l2 volts at the top of diode D1, there is nocurrent owing through diode D1 in either its forward or reversedirection. This arrangement provides that there be an approximate 20volts drop across resistors R1 and R2 and if variable resistor R2 is setat 500 ohms, which is recommended, then a current of 20 milliampereswill flow through these two resistors since current flow is prohibitedthrough diodes D1 and D2. Therefore, the current must go throughresistor R3 and then divide between diode D3 and resistor R5.

The value of diode D3 is chosen so that approximately a 6 volt drop willappear across the diode and so that approximately a 6 volt drop appearsacross resistor R5 having 6 milliamperes passing therethrough. Theremaining 14 milliamperes passing through will pass through diode D3.This current is well within the rating of diode D3 and yet it is highenough so that a small change in current will not affect the 6 volt dropacross the diode' anl the voltage at the top of diode D3 willy remain at6 vo ts.

When the D.C. voltage across input terminals 15 and 16 changes from 32volts to 33 volts, solenoid 11 of relay 10 will be energized.Energization is accomplished as follows: Inasmuch as the combination ofdiode D1 and diode D2 produced a voltage drop of 12 volts, the voltageacross resistor R3 will not appreciably change so that the currentthrough diode D3 and resistorl R5 does not change. However, theincreased voltage causes a larger drop across resistor R1 and R2 whenconsidering diode D1 and D2 theoretically. The voltage drop remainsconstant at l2 volts and the one volt increase is sensed across resistorR1 and resistor R2 whereby the current passing therethrough increasesproportionately. Since no more current goes through resistor R3, the onemilliampere increase passes through diodes D1 and D2 and through thetemperature compensating circuit 18 comprising resistors R6, R7, and R8.This one milliampere of current will produce a voltage drop of about onevolt across the temperature compensation circuit and this voltage issuicient to pass current through base circuit 2t) of transistor 14.

The circuit of the present invention is designed so that there isapproximately 12 volts at the top of diode D1; however, inasmuch ascommercially available diodes are not theoretically perfect, thenon-conductance point could be 11% or 121/2 or vary anywhere in between.The purpose of resistor R2 is to correct for these variations so thatthe diodes will not have to be selectively assembled and chosen andtherefore R2 can be adjusted to compensate for all the required voltagedrops at a voltage just below the solenoid energizing voltage.

Transistor and Relay Operation Current normally passing through solenoid11 of relay passes from input terminal 15 to the relay solenoid viaresistor R9 and then to but not through diode D6 to a collector circuit19 of transistor 14 and then through transistor emitter circuit 21 toground 17. The amount of current that passes through the relay iscontrolled by the impedance of the transistor which in turn isdetermined by the transistor base current flowing in the base circuit20. Thus, the one volt at the top of resistor R6 causes relay current toilow and causes solenoid 11 to be energized. Once the solenoid commencesto be energized, relay contacts X-1 and X-2. open which couples resistorR4 in the diode network. This results in a voltage rise at diode D1 andeliminates the possibility of relay contact chatter.

When the relay is not energized, contact 13 shorts out resistor R4through leads X-1 and X-Z. When the solenoid commences to energize dueto a reduced transistor impedance, resistor R4 is no longer shorted butit is placed in the diode network in series with diode D3. Inasmuch asabout 14 milliamperes are passing through the diode, there is a voltagedrop of more than one volt across resistor R4. This increases thevoltage at diode D1 by about one volt and increases the current throughdiodes D1 and D2 by approximately a ratio of 5 to 1. The effect is tocause more transistor base current to flow and th-us more relay currentwill be present which will positively eliminate relay chatter.

Time Delay Network A time delaying network is provided which comprisesR9, R10, and R11 and capacitors C1, C2 and C3 and diodes D4 and D5.Assuming a condition before relay energization when no current isilowing in the relay coil, the entire 32 volts applied across inputterminals 15 and 16 are across the transistor and there is no voltagedrop across the time delay network. When the input voltage is increasedto 33 Volts or more, the transistor impedance decreases and there is avoltage drop across the relay. Thus, the voltage of collector circuit 21of the transistor is somewhat below 32 volts and a voltage drop existsacross the time delay network and current will -ilow through thecapacitors.' This current is required to pass through resistor R9causing a voltage drop to exist momentarily which decreases the voltagedrop across the relay. This is the time delay in the circuit and itlasts until capacitors C1, C2 and C3 are charged to the equivalent timedelay at various input voltages. Obviously, the limits of time delaydepend on the values of the various resistors and capacitors in thedelay network. The particular delay employed in the present embodimentwas designed for a particular application and it is to be understoodthat the delay can be varied for any particular time required lby otherapplications. A feature exists in employing this time delay networkwherein the voltage drop across the time delay network is the samewhether the input voltage between input terminals 11 and 12 is volt or32 volts. Thus, the capacity of the delay network to absorb a charge isthe same regardless whether the starting voltage is 0 or 32 volts.

Temperature Compensation Inasmuch as the parameters of transistors varyconsiderably at various ambient temperatures, the resultant differenceis that a different base current is required for an equivalent change ofimpedance. Thus, at low ambient temperatures more base current isrequired, and at high ambient temperatures less base current is requiredthan for an equivalent operation at normal or room temperature. Themethod to provide this higher base current is to employ a higher Voltageat the top of resistor R6. This is accomplished by employing thetemperature compensation circuit 18 between resistor R6 and ground 17.The temperature compensating circuit comprises a pair of thermistors 30=and 25 coupled across resistors R7 and R8 which provide `a propervoltage drop. The temperature compensation circuit representsapproximately 1,000 ohms of resistance at 25 centigrade.

Reverse Voltage Operation The present invention further includes areverse voltage network so that when a reverse voltage between inputterminals 15 and 16 is present, the relay will operate. This means thatif input terminal 16 is positive and input terminal 15- is negative, therelay will be energized. The relay should be fully energized at a lowervoltage than if input terminal 15 were positive. The relay should close-at approximately 18 to 20 volts and this is accomplished by shuntingacross transistor 14 with diode D6 and shunting resistor R9 with diodeD7. Thus, if ground 24 is positive, current will ilow directly from thisground point through ground 17 and relay solenoid 11 and through diodeD7 to input terminal 15. The capacitors C1, C2 and C3 are protected fromthe reverse Voltage by diode DS which will not permit current to flowthrough the capacitors in a reverse direction. However, since thecapacitors will have to discharge in a reverse direction across diodeD5, means are provided which allow the capacitors to discharge eventhrough diode D5 is in the circuit. This means comprises diode D4 andresistor R11 wherein resistor R11 should be of suiiicient value so thatreverse voltage will not damage the capacitors and yet low enough valueso that the capacitors will discharge in the direction from capacitor CZto resistor R10 via relay solenoid 11 and then to resistor R11, diode D4and back to capacitance C2.

Remarks In put terminal 29 is connected between resistors R1 and R2which serves as a test circuit for the over voltage relay. The relayshould close at about 22 to 25 volts D.C. and the test circuit may beemployed to determine if the over voltage relay is functioning properlyand if the circuitry `which the over voltage relay controls is alsofunctioning properly. Terminals 26 and 27 comprise a normally closedcircuit which when opened can be used to disrupt the generator powersource (not shown) which the relay protects while a terminal 28 incombination with terminal 26 can be used to disrupt the generator powersource when closed.

The relay employed on one embodiment of the present invention is asigma-type 2l-RJCC with a 5,000 ohm coil. The high coil resistance isrequired because this resistance must be high in relation to lthe lowestimpedance sensed across the transistor. The transistor impedance`generally drops to about 1,000 to 1,500 ohms and the 5,000 ohm coilresistance is reasonably high with respect to that impedance. Forillustrative purposes, a transistor manufactured fby the TexasInstrument Company, catalogue number 951, has been employed and appearsto have suicient sensitivity.

'Features in the present invention reside in the fact that the relaywill not energize until the input D.C. voltage is within approximatelyone volt of relay energization and furthermore .the relay is not fullyenergized until the exact pull-in point at one-half volt below pull inof the relay network has been achieved.

For convenience of actual construction, the following values arerecommended lfor the various components of the system:

Resistors and Capacitors Plus or M inus 10% Unless Noted R1 500 ohms.

R2 1,000 ohms potentiometer.

R3 340 ohms, plus or minus 5%.

R4 100 ohms.

R6 200 ohms.

R7 1,500 ohms.

R8 1,500 ohms.

R9 2,000 ohms.

R10 600 ohms.

R11 5,000 ohms.

C1, C2, C3 100 afd.

D1, D2 |1N429, National Semi- Conductor Products.

D4, D5, D6, D7 1N91.

-Having described only typical forms of the invention I do not Wish tobe limited to the specific details herein set forth, but wish to reserveto myself any variations or modifications that may appear to thoseskilled in the art and fall within the scope of the following claims.

I claim:

1. An automatically operating D.C. over voltage protecting device forcontrolling an electrical apparatus by switching olf power to Saidapparatus in response to over voltages occurring in said apparatuscomprising in comlbination:

I. a switching relay circuit containing the following elements in serieswith one another 'between input voltage terminals;

A. the coil of a normally de-energized switching relay having rst andsecond sets of contacts,

B. a controlling transistor connected rby collector and emitter forcontrolling current through said relay coil,

C. a unidirectional conducting diode in reverse position with respect tonormal current direction and connected across said collector and emitterof said transistor,

II. a transistor base controlling circuit connected in parallel to thetotal circuit of said switching relay circuit and containing in series;

A. at least one variable resistive element,

B. a temperature compensating diode circuit having unidirectionalconducting diodes arranged in reverse position with respect to normalcurrent direction and having a connection to the ibase of saidtransistor extended from the reverse side of the -last of said diodes,

C. said temperature compensating diode circuit containing in seriesafter said transistor base connection at least one resistor and at leastone thermistor in parallel with said resistor,

D. a Iby-pass circuit for fby-passing current around and in parallelwith said diode circuit when normal operating Voltage is impressed fromsaid apparatus load and containing a normally shunted out resistiveelement,

III. a time delay circuit arranged in parallel with said switching relaycircuit for momentarily thy-passing current around said relay coil toprovide for a time lapse in said relay actuation containing;

A. a plurality of capacitors arranged in parallel,

B. unidirectional conducting diodes arranged in reverse relationship toone another in separate parallel legs Iboth in series with saidplurality of capacitors,

IV. all of said unidirectional conducting `diodes having fthecharacteristic of passing current freely in one direction and passingcurrent in the opposite direction only after a predetermined voltage isimpressed thereupon without destruction,

V. said first set of relay contacts controlling the power to saidapparatus, and

VI. said shunted out resistive element controlled by said second set ofrelay contacts for adding said shunted out resistive element to saidThy-pass circuit simultaneously with controlling of power to saidapparatus so that greater current is impressed upon said transistorrbase which in turn increases relay current through said transistorwhich controls chattering of said relay contacts.

2. An automatically operating D.C. over voltage protecting device forcontrolling an electrical apparatus by controlling power to saidapparatus in response to over voltages occurring in said apparatuscomprising in combination:

I. a switching relay circuit containing the following elements in serieswith one another between input voltage terminals;

A. the coil of a normally de-energized switching relay having at leastone set of contacts,

B. a controlling transistor connected by collector and emitter forcontrolling current through said relay coil,

C. a unidirectional conducting diode in reverse position with respect tonormal current direction and connected across said collector and emitterof said transistor,

II. a transistor Abase controlling circuit connected in parallel to thetotal circuit of said switching relay circuit and containing in series;

A. at least one variable resistive element,

B. a temperature compensating diode circuit having unidirectionalconducting diodes in reverse position with respect to normal currentdirection and having a connection to the base of said transistorextended from the reverse side of the last of said diodes,

C. said temperature compensating diode circuit containing in seriesafter said transistor base connection at least one resistor and at leastone thermistor in parallel with said resistor,

D. a by-pass circuit for by-passing current around and in parallel withsaid diode circuit when normal operating voltage is impressed from saidapparatus load and containing a resistive element which is balanced withrespect to said diode of said diode circuit so that voltage greater thana predetermined value will allow said diode to conduct current andthereby control said transistor amplification and in turn said relay forcontrolling power to said load,

III. a time delay circuit arranged in parallel with said relay circuittfor momentarily by-passing current around said relay coil to providefor a time lapse in said relay actuation and containing a plurality ofcapacitors arranged in parallel,

IV. all of said unidirectional conducting diodes having thecharacteristic of passing current freely in one `direction and passingcurrent in the opposite direction only after a predetermined voltage isimpressed thereupon without destruction, and

V. said iirst set of relay contacts controlling the power to saidapparatus.

References Cited in the iile of this patent UNITED STATES PATENTS2,548,818 Rambo Apr. l0, -1 2,769,131 Immel Oct. 30, 1956 2,801,374Svala July 30, 1957 2,804,578 Bergseth Aug. 27, 1957 2,816,262 ElliottDec. 10, 19-57 2,828,450 Pinckaers Mar. 25, 1958 2,866,106 Schuh Dec.23, 1958 OTHER REFERENCES Transistorized Headlight Dimmer, August 1955;3 pages.

Temp. Compensation Method 'for Transistor Ampliiers, November 15, 1956;6 pages.

